Shear wave elastography has been known for several years as an efficient technique for detecting an inhomogeneity of elasticity in a soft solid, such as a tumor. This technique is based on the detection of shear waves propagation speed. Such a detection can be based on an ultrasonic technology or on a magnetic resonance imaging (MRI) technology.
In a soft solid, shear waves propagate at a speed in the range of 1 to several meters per second (m/s) and this speed can be used to characterize a target region of a soft solid since the speed pattern of these waves allows generating images representative of the shear elastic modulus. This shear elastic modulus approximately corresponds to the elasticity which can be sensed by palpation and is ranging from a few hundred Pa to a few thousand kPa. This is different from an approach based on the propagation of compression waves which propagate at a much higher speed, in the range of 1500 m/s, in a soft solid. The repartition of the compression waves in such a solid is representative of the compression elastic modulus of the tissue, typically in the order of 2.4 GPa, six orders of magnitude bigger than the shear elastic modulus. This is why biological tissues are generally considered not to be compressible. Thus, a detection method based on the propagation of compression waves cannot be considered as an elastography method.
In the field of elastography, it is known from U.S. Pat. No. 6,770,033 to use a loudspeaker controlled by a micro computer so as to apply an excitation, in the form of a low-frequency pulse, to the surface of a soft solid. In order to use the device of U.S. Pat. No. 6,770,033, one must manipulate, with one hand, the loudspeaker and, with the other hand, a sensor which is supposed to collect information with respect to the propagation of shear waves generated in the tissue. This is tedious and requires a high expertise.
U.S. Pat. No. 5,606,971 discloses a method for shear wave elasticity imaging where shear waves are generated in a focus zone of a piezo-electric transducer inside soft solids to be studied. This transducer has a double function: it creates the shear waves and it detects them. With this approach, shear waves can be generated in a target zone close to the transducer array. However, this technique is difficult to implement when the target zone is far from the transducer array, for instance in deep organs. Actually, the ultrasound energy deposition quickly decreases with depth, and under such circumstances, the ultrasound radiation force cannot create effective shear waves propagation.
On the other hand, US-A-2009/018432 discloses a method for imaging with magnetic induction, where one uses a magnetic pulsed stimulation with a duration in the order of one microsecond (μs). In other words, the excitation frequency of the magnetic stimulation is in the range of 1 MHz, which generates compression waves, but no shear waves. It is known that shear waves are strongly attenuated and cannot propagate in tissues at such a frequency. A key aspect of the teachings of US-A-2009/018432 is that the precision of the resolution depends on the frequency used for the magnetic stimulation. Under such circumstances, the method described in this document relies on the use of high frequency excitation, in order to obtain a satisfactory resolution. On the contrary, elastography requires low vibration frequencies to induce shear waves. Therefore the technology of US-A-2009/018432 is not adapted to shear wave elastography.
U.S. Pat. No. 6,583,624 discloses a method where a voltage applied to a subject material is used to produce shear waves and an IRM technique is used to detect the shear waves. A magnetic field is used for the IRM detection, but it does not participate to the shear waves generation.
US-A-2006/0152219 discloses a method where either a piezoelectric actuator, alternating currents or an alternating magnetic field is/are used to input mechanical motions to spins within a sample.
In these two last prior art documents, one relies on a single electric phenomenon to move some particles and, where a magnetic field is applied for the IRM detection, it is not used to generate shear waves. Thus, shear waves generation in deep target regions is also difficult to obtain with these techniques.